An electronic transformer is an AC electronic component that will change, or transform an AC input voltage to a different output voltage level. An important characteristic of typical transformers is that circuitry connected to the primary is electrically isolated from circuitry connected to the secondary winding. An output voltage higher than the input voltage will generate a lower output current, and a lower output voltage will generate a higher output current. After accounting for losses in the transformer the power into the transformer is substantially equal to the output power produced. A transformer may have a first, primary winding upon a core, with a second winding, or secondary, also disposed upon the same core. The primary core to which an input voltage is applied, through electromagnetic coupling induces a voltage across the secondary. Accordingly the output voltage of a transformer may be changed by adding or removing secondary turns
Alternatively, discrete voltages may be selected by attaching wires (taps) at various taps. The taps if connected to a rotary switch provide discrete, but variable output voltages.
In an alternative construction a more continuous output voltage may be produced by allowing a conductor (typically a carbon brush), to slide over exposed turns of a secondary winding. Typically, a knob is provided, and turning it in one direction increases the voltage output, and the opposite direction decreases the output voltage.
Transformers of this sort may be desirable in applications which require a variable voltage, such as light dimmers, welders, motor controls, audio applications, testing equipment at low and high end operating conditions, and the like. However, using a conventional transformer with a bulky core and two windings in such applications would not be practical. If electrical isolation is not needed a device called an autotransformer may be substituted for a transformer. It advantageously utilizes a single winding in which taps or brushes may be applied as previously described in a transformer.
FIG. 1. shows a schematic of an autotransformer 100, which has a single winding 102 over a core material 104 with two primary terminals 106 and 108 at the extreme ends of that single, or primary winding. It also has one or more terminals or taps 110 at intermediate tap points along the single winding 102 that forms the secondary winding or circuit. Thus the primary and secondary coils have part or all of their turns in common.
The primary voltage 112 is applied across two of the primary terminals, and the secondary voltage 114 taken from the tap terminals. The autotransformer almost always has one terminal 108, in common with the primary voltage. The primary and secondary circuits, therefore, have a number of windings turns in common. Since the volts-per-turn is the same in both windings, each develops a voltage in proportion to its number of turns. In an autotransformer, part of the current flows directly from the input to the output, and only part of the current is transferred by induction.
Autotransformers may also include many taps and include additional automatic switchgear to allow them to act as automatic voltage regulators to maintain a steady voltage over a wide range of load conditions. If a sliding tap is used that contacts more than one turn at a time, the turns are shorted. However if a resistance is inserted sliding tap the shorting problem may be eliminated. An autotransformer that is designed to produce continuous voltage variation, without shorting adjacent turns is known as a variable autotransformer, such as the VARIAC® variable autotransformer from Instrument Service and Equipment, Inc., Cleveland, Ohio
FIG. 2 shows an electrical schematic of a variable autotransformer. In a variable autotransformer, part of the winding coils 202 may be exposed and the secondary connection is made with a sliding brush 204. The brush is typically a carbon brush. The primary connection is 206. The addition of the brush, which may be controlled with an external knob (not shown) allows a continuously variable turns ratio to be obtained, which is established by the location in the winding the brush makes contact. This allows for very smooth control of voltage. The output voltage 208 is not limited to the discrete voltages represented by actual number of turns. The input voltage 210 can be smoothly varied between turns as the brush has a relatively high resistance (compared with a metal contact) and the actual output voltage is a function of the relative area of brush in contact with adjacent windings. The primary connection 206 can be connected to only a part of the winding allowing the output voltage to be varied smoothly from zero to above the input voltage. This allows a variable autotransformer to be used for testing electrical equipment at the limits of its specified voltage range.
Brushes make physical and electrical contact in conducting electricity between moving parts and tend to wear from use. Typical applications of brushes include electric motors, alternators, electric generators, and variable autotransformers. Accordingly it would be desirable to eliminate the use of brushes in a variable transformer design.
Those having skill in the art would understand the desirability of having a variable transformer that uses circuitry to vary and regulate output voltage without brushes. The variable transformer described herein allows the use of a variable transformer not requiring cleaning and maintenance of moving parts, nor mechanical brushes.